The pollution of ground and drinking water by metalloids, especially arsenic, a wide range of toxic metals, such as but not limited to, lead, mercury, cadmium, chromium, nickel, iron, cupper, platinum, and palladium, as well as various organic chemicals, has attracted increasing attention in recent decades throughout the world. Particularly, the contamination of water by arsenic has been one of the most serious health hazards and its remediation to the Level of Concern (LOC) (10 ppb) has proven challenging.
The presence of arsenic in waterways is due to both natural and anthropogenic causes. For example, arsenic may be released into waterways due to volcanic activity and from erosion of natural deposits such as rocks and terrain that has been burned due to forest fire. Further, arsenic may be released into waterways due to agricultural runoff, industrial production waste runoff, etc. For example, some fertilizers contain arsenic and further, industrial practices such as copper smelting, mining, and coal burning also contribute to arsenic in the environment.
Consuming excessive amounts of arsenic from drinking water may contribute to a number of mild to severe health effects. For example, arsenic has been linked to thickening and discoloration of the skin, stomach pain, nausea, vomiting, diarrhea, numbness of the extremities, partial paralysis, and blindness. Further, arsenic has been credited as a carcinogen and linked to cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate, as well as a factor contributing to cardiovascular disease.
Since ground water sources and surface water sources are susceptible to arsenic contamination, it is imperative to purify water from these sources prior to human and animal consumption. The World Health Organization (WHO) and US Environmental Protection Agency (EPA) have set the arsenic standard for drinking water at 0.010 parts per million (10 ppb) to protect consumers served by public water systems from the effects of long-term, chronic exposure to arsenic. While the EPA standard applies to municipal water treatment facilities, it is desirable to remove arsenic from other water treatment systems as well. Indeed, in the United States, the National Resources Defense Council estimates that over 34 million Americans drink from water supplies with average arsenic concentrations that pose unacceptable cancer risks.
Arsenic removal technology can be applied to large scale water treatment systems, small scale water treatment systems, point-of-use water treatment systems, well water treatment systems, portable water treatment systems, and other systems.
Previous solutions for removing arsenic from drinking water involve processes/technology such as flocculation, modified coagulation/filtration, modified lime softening iron oxide adsorption, activated alumina, ion-exchange, reverse osmosis, electrodialysis, subterranean arsenic removal (SAR), and metal loaded polymers. Flocculation and iron oxide adsorption techniques generally use an iron-based coagulant to remove arsenic by co-precipitation and/or adsorption. However, the toxic arsenic sludge resulting from coagulation often clogs the system and the toxic arsenic sludge has to be disposed of by concrete stabilization. While this may be a sufficient short term solution, the toxic arsenic sludge may leach over time and thus be reintroduced into the environment.
Ion-exchange has traditionally been used as a water-softening process and has some ability to remove arsenic. However, arsenic exists in two oxidation states in water depending on the oxidation-reduction conditions and the pH of the water. As(III) is usually associated with groundwater under anaerobic conditions, while As(V) is associated with surface water under aerobic conditions. As(III) is found as the neutral species, arsenous acid (H3AsO3), below pH 9. As(V) occurs as the monovalent and the divalent arsenate species, H2AsO4− and HAsO42−, respectively, between pH 6 and 9. Ion-exchange is ineffective in removing non-charged arsenic(III) species. Further, the presence of sulfate and high total dissolved solids can significantly affect run length and maintaining an ion-exchange column is costly and requires a skilled technician.
Reverse osmosis and electrodialysis techniques can remove arsenic but result in high salinity waste water, which presents an issue in that it requires further waste water treatment. Further, both technologies are high cost. SAR technology utilizes an oxidation zone to trap iron and arsenic underground. The technology relies upon soil dwelling microorganisms to metabolize iron and arsenic and break these substances down to other molecular species. Such a technology is extremely expensive to develop and operate. In addition, this technique is not simple and requires well simulated (calculated) and balanced aquifer oxidation. Otherwise, the oxidation procedure will just lead to arsenic and iron co-precipitation rather than adsorption, resulting in the subsequent release of arsenic. Metal-loaded polymers and granular metal, especially Fe(III) are interesting due to the possibility to remove both As(III) and As(V), however, the arsenic binding is pH dependent and there remains the possibility of releasing the impregnated metal in solution and adversely affecting the quality of drinking water